When soldering electronic components on a board, a reflow furnace is generally used. The reflow furnace is provided with a conveyor for conveying the board and a reflow furnace main body (a muffle) with a tunnel shape, to which the conveyor conveys the boards . On the interior of the reflow furnace main body, a preheating zone, a main heating zone and a cooling zone are provided along a conveying path from a receiving entrance to a discharging exit . A pair of a hot-air-blowing heater and a fan is set on each of the preliminary heating and heating zones. The hot-air-blowing heater and the fan are set on respective portions above and below the conveyor. Fans for blowing cooled air that cools down the printed circuit board, which has been heated in the preheating zone and the main heating zone, and motors which rotates the fans are set on the cooling zone.
The board on which soldering paste has been previously printed is conveyed into the reflow furnace in the reflow processing. The soldering paste printed on the board contains powdered solder, solvent and flux. Among them, the flux contains rosin or the like as its component so that the flux has an effect to remove an oxide film from a metal surface to be soldered, to prevent reoxidation by applying heat thereto when soldering and to make surface tension of the solder smaller so as to become improved in wettability thereof.
From the board conveyed in the reflow furnace, the solvent contained in the soldering paste is volatilized in the preheating zone. Next, in the main heating zone, heated wind is blown to the board which is conveyed by the conveyor from the upper and lower directions so that the soldering paste is fused. In the cooling zone, the heated board is then cooled and the fused solder is solidified. Under such a series of steps, the electronic components are soldered on any junctions on the printed circuit board.
Incidentally, in the above-mentioned heating step, the flux is evaporated by applying heat thereto with the heater and is filled in the reflow furnace main body. The flux filled in the reflow furnace main body is generally cleaned via a removing apparatus for removing a flux component, which is set outside the muffle, and is then again returned to the muffle via a circulation path. However, since, accompanying the rotation of the fans set on the preheating zone, the main heating zone and the cooling zone, a flow flown into a back side of each of the fans occurs, a part of the evaporated flux may be flown into the back side of each of the fans by this flow. The evaporated flux which is flown into the back side of each of the fans is here cooled to become liquid flux having any fluidity. When the rotation of the motor stops and internal temperature thereof drops after the evaporated flux becomes liquid flux, viscosity of the liquid flux is increased and its fluidity drops so that the flux is solidified and accumulated on a base portion thereof. In this moment, a problem occurs such that when the flux is solidified on the rotation axis of each of the fans, the rotation of each of the fans is obstructed.
In order to prevent the flux from being accumulated, a reflow furnace which is provided with an inclined portion that is inclined from an axis of the fan to its circumference and a gas inlet for inserting the gas having a temperature which is capable of liquefying the flux on a bottom of a casing of the reflow furnace has been proposed (see Patent Document 1). Flux collection openings are provided on two positions on a periphery of the bottom of the casing of the reflow furnace so that the flux on the bottom is flown along the inclined surface and flown into the flux collection openings, thereby preventing the flux component from being accumulated.